The invention relates to a method for stabilizing the rotation speed of a hydraulic machine with S-characteristics. Typical hydraulic machines with S-characteristics are pump-turbine functioning in turbine mode or Francis turbine designed for high net head conditions. The invention also concerns an installation for converting hydraulic energy into electrical energy, on which this method can be implemented.
During a pump-turbine start-up in turbine mode, the rotation speed of its runner must be stabilized and synchronized with the grid frequency, so that the machine can be coupled to the grid. Besides, the coupling operation is usually performed when the pump-turbine is at a no-load operating point, at which the water flow does not exert any torque on the runner. This particular operating point is the coupling point.
Under low water fall conditions, rotational speed stabilization may be hard to reach because of the presence of “S-zones”. “S-zones” are unstable zones wherein the machine is sensitive to perturbations. As a result, a slight rotational speed variation relative to the coupling point leads to a significant increase of the torque applied by the water flow on the runner, hence substantially increasing or decreasing the rotation speed of the machine. Under these conditions, it is not possible to stabilize the machine rotational speed with a conventional PID loop.
To solve this problem, it is known to completely redesign hydraulic parts of the machine, such as the runner or the guide vanes. In particular, hydraulic parts of the machine are redesigned to avoid the presence of S-zones in the operating range of the pump-turbine in turbine mode. The operating range of a machine corresponds to the interval between the lower gross head and the upper gross head to which may be subjected the machine. However, this solution is very expensive to implement and reduces the performance of the pump-turbine.
Another solution consists in equipping the pump-turbine with non-synchronized guide vanes. This means that some guide vanes can be oriented independently. As a result, at machine start-up, some of the guide vanes are opened further than others, which temporarily modifies the machine characteristics. This allows avoiding the presence of S-zones during start-up. Nevertheless, this solution generates unwilled vibrations which impact the life-time of the machine.
Finally a method for stabilizing the rotation speed of a turbine apparatus at a point that is very close to the S-characteristic portion, without risking entering in this unstable portion. The S-characteristic portion can be defined as the curve portion with a positive slope on FIG. 1A of the application, that is the portion below the NR curve. The NR curve is the curve at which no torque is exerted by water flow on the turbine runner. The method consists in using a PID controller acting directly on the rotation speed of the machine. This PID controller is specific in that its characteristic parameters may be changed depending on the rotation speed of the machine. More precisely, when the machine rotation speed increases from 0 to a predetermined speed, a high gain is affected the coefficients of the proportional and integrative elements of the PID controller. However, when the rotation speed exceeds the predetermined speed, a low gain is affected to the coefficients of the proportional and integrative elements of the PID controller.
In the described example, the predetermined speed is set to 80% of the target rotation speed. As a result, when the machine operating point gets close to the coupling point, the speed of convergence toward the coupling point is slowed down to avoid exciting the system and entering into the unstable S-characteristic portion. The characteristic parameters of the PID controller are selected between two values using a pair of switches.
The major drawback of this method is that it does not allow stabilizing the rotation speed of the machine when the coupling point is precisely located into the S-characteristic portion.